Button cells normally have a housing consisting of two housing half-parts, a cell cup and a cell top. By way of example, these may be produced from nickel-plated deep-drawn metal sheet as stamped and drawn parts. The cell cup normally has positive polarity, and the housing top negative polarity. The housing may contain widely differing electrochemical systems, for example, zinc/MnO2, primary and secondary lithium systems, or secondary systems such as nickel/cadmium or nickel/metal hydride.
By way of example, rechargeable button cells based on nickel/metal hydride or lithium-ion systems are in widespread use. In the case of lithium-ion button cells, the electrochemically active materials are normally not arranged within the button cell housing in the form of individual electrodes, in the form of tablets, separated from one another by a separator. Instead, prefabricated electrode-separator assemblies are preferably inserted flat into the housing. In that case, a porous plastic film is preferably used as a separator, onto which the electrodes are laminated or adhesively bonded flat. The entire assembly comprising the separator and the electrodes generally have a maximum thickness of a few hundred μm. To allow button cell housings of normal dimensions to be filled, a plurality of such assemblies are therefore frequently placed flat one on top of the other. This allows stacks of any desired height, in principle, to be produced, in each case matched to the available dimensions of the button cell housing into which the stack is intended to be installed. This ensures optimum utilization of the available area within the housing.
By virtue of the design, however, various problems also occur in the case of button cells which contain such stacks of electrode-separator assemblies. On the one hand, it is necessary, of course, for the electrodes of the same polarity each to be connected to one another within the stack, and then each to make contact with the corresponding pole of the button cell housing. The required electrical contacts result in material costs, and the space occupied by them is, furthermore, no longer available for active material. In addition, the production of the electrode stacks is complicated and expensive since faults can easily occur when the assemblies make contact with one another, increasing the scrap rate. On the other hand, it has been found that button cells having a stack of electrodes and separators very quickly start to leak.
Traditionally, button cells have been closed in a liquid-tight manner by beading the edge of the cell cup over the edge of the cell top in conjunction with a plastic ring, which is arranged between the cell cup and the cell top and at the same time acts as a sealing element and for electrical insulation of the cell cup and of the cell top. Button cells such as these are described, for example, in DE 31 13 309.
However, alternatively, it is also possible to manufacture button cells in which the cell cup and the cell top are held together in the axial direction exclusively by a force-fitting connection, and which do not have a beaded-over cup edge. Button cells such as these and methods for their production are described in German Patent Application 10 2009 017 514. Irrespective of the various advantages which button cells such as these without beading may have, they can, however, not be loaded as heavily in the axial direction as comparable button cells with a beaded-over cup edge, in particular with respect to axial mechanical loads which are caused in the interior of the button cell. For example, the electrodes of rechargeable lithium-ion systems are continually subject to volume changes during charging and discharging processes. The axial forces which occur in this case can, of course, lead to leaks more readily in the case of button cells without beading than in the case of button cells with beading.
It could therefore be helpful to provide a button cell in which the problems mentioned above do not occur, or occur only to a greatly reduced extent. In particular, it could be helpful to provide a button cell that is resistant to mechanical loads which occur in the axial direction than conventional button cells, in particular even when they are manufactured as button cells without a beaded-over cup edge.